Most semiconductor chips used in electronic devices are fabricated from single crystal silicon prepared by a Czochralski-type (CZ) process. In the CZ process, a single crystal silicon ingot is produced by melting polycrystalline silicon source material in a crucible, stabilizing the crucible and source melt at an equilibrium temperature, dipping a seed crystal into the source melt, withdrawing the seed crystal as the source melt crystallizes on the seed to form a single crystal ingot, and pulling the ingot as it grows. Melting occurs at a temperature of 1420° C. in an inert gas environment at low pressure. The crucible is continually rotated about a generally vertical axis as the crystal grows. The rate at which the ingot is pulled from the source melt is selected to form an ingot having a desired diameter.
The polycrystalline silicon can be prepared using a fluidized bed reactor process to form granules. Alternatively, the polycrystalline silicon can be prepared using a chemical vapor deposition (CVD) process in a bell jar reactor. The polycrystalline silicon prepared by the CVD process can be broken or cut into suitably sized pieces such as rods, chunks, chips, and combinations thereof, before loading into the crucible. The polycrystalline silicon is melted to form the molten silicon.
One of the drawbacks of the CZ process is that when the charge of polycrystalline silicon is melted, the crucible may be only half full of molten silicon. This is due to the interstitial spaces left in the crucible charged with irregularly shaped pieces and results in inefficient utilization of the crystal puller. Therefore there is a need to develop methods to efficiently top up a charge after it is melted and before the start of the crystal seeding.
A further drawback of the CZ process is that the crucible can generally be used for only one pull before it must be replaced because crucibles degrade with use and can introduce impurities into the molten silicon. New crucibles are expensive to obtain, and used crucibles are expensive to dispose of. This has led to development of improved crucibles capable of lasting through multiple ingot pulls while contributing reduced contamination to the molten silicon. Therefore, a need exists for efficiently recharging the crucible during or after pulling the first ingot and any subsequent ingots. Various methods for topping up melts and recharging the crucible have been proposed.
In one method, granular polycrystalline silicon made by a fluidized bed process (such as granular material made by Ethyl Corporation or MEMC) has been loaded into the molten heel remaining in the crucible after the ingot has been pulled or to top up the initial charge melt. However, this method suffers from the drawback that granular polycrystalline silicon made by the fluidized bed process contains entrapped hydrogen. When the granular polycrystalline silicon is added to the heel, the hydrogen is released, causing the granules to burst. This causes splashing of molten silicon, which can damage the crucible.
In another method, granular polycrystalline silicon is added to the crucible while the ingot is being pulled. However, this method suffers from the drawback that due to its small particle size, granular polycrystalline silicon is difficult to melt in sufficient time to achieve a reasonable addition rate. Additional heat is required to melt these small particles, leading to added cost and accelerated crucible degradation. Accelerated crucible degradation can shorten crucible life and increase cost. If the granular polycrystalline silicon addition rate is too fast and the granules do not melt sufficiently, this can damage the surface of the ingot being pulled and cause dislocations and damage singularity of the crystal. Furthermore, granular polycrystalline silicon may have high amounts of dust. Dust can create contamination problems in the puller housing and can move to the surface of the pulled ingot and cause dislocations and reduce crystal yields. This may also increase process time, due to the need to remelt and repull the ingot.
Overall, granular polycrystalline silicon has inadequate purity for some applications, regardless of the process used to recharge the granules.
Previous attempts to use polycrystalline silicon rods prepared by a chemical vapor deposition process and broken into pieces have also not been used for crucible recharge due to purity or size problems. If relatively large size polycrystalline silicon pieces are used for crucible recharge, the process may suffer from the drawbacks of damage to the crucible and damage to the recharge apparatus. If the polycrystalline silicon pieces are broken into smaller sizes, contamination with impurities has made the polycrystalline silicon pieces unsuitable for use in crucible recharge processes.